Disc drives read and write information along concentric tracks formed on discs. To locate a particular track on a disc, disc drives typically use embedded servo fields on the disc. These embedded fields are utilized by a servo sub-system to position a head over a particular track. The servo fields are written onto the disc when the disc drive is manufactured and are thereafter simply read by the disc drive to determine position.
Ideally, a head following the center of a track moves along a perfectly circular path around the disc. However, two types of errors—known as repeatable runout (RRO) and non-repeatable runout (NRRO) prevent heads from following this ideal path. A position error is considered to be RRO if the same error occurs each time the head passes a particular circumferential location on the disc. The portion of the total position error that is not RRO is considered NRRO. This invention relates to reducing errors associated with RRO. RRO can further be decomposed into two types. The first type of RRO error is a written-in error that arises during the creation of the servo fields. Written-in errors occur because the write head used to produce the servo fields does not always follow a perfectly circular path due to unpredictable pressure effects on the write head from the aerodynamics of its flight over the disc, and from vibrations in the gimbal used to support the head. Other sources of written-error exist. Because of these written-in errors, a head that perfectly tracks the path followed by the servo write head will not follow a circular path.
The second type of RRO error that prevents circular paths is induced repeatable runout. Briefly, RRO is induced during the head-disk assembly (HDA) build process due to disk clamping distortion and spindle runout. The level of induced RRO can be quite large. Therefore, the total level of RRO (written-in and induced) in drives with prewritten media may exceed the RRO budget, requiring some form of RRO compensation.
Written-in error compensation techniques are commonly used within the disk drive industry to reduced servo tracking errors associated with RRO. Some compensation techniques involve a factory calibration process to generate correction tables for each track. These tables are written to the disk and used by the servo subsystem to compensate for the RRO, thereby reducing tracking errors. This factory process can be time consuming and costly, requiring several hours of factory calibration time per drive.
A further factor to be considered is the increasing tracks per inch (TPI) of disc drives. To meet the demands of higher TPI capability, servo-track writing is undergoing a fundamental change: In the near future, servo tracks will be prewritten or preformatted outside the HDA. One such method would be the use of a multi-disk writer (MDW). The written-in level of RRO on prewritten or preformatted media may be sufficiently small so that the compensation techniques described above are not required, resulting in considerable time and cost savings. However, as explained above, the additional induced RRO can be quite large. Therefore, the total level of RRO (written-in and induced) in drives with prewritten media may exceed the RRO budget, requiring some form of RRO compensation. If die compensation techniques describe above are used to bring the total RRO down to the required level, the factory calibration process used for prewritten or preformatted media would remain costly.